Bioassay of pendimethalin at different moisture levels in wheat for controlling Polygonum

The terai agro-climatic region of West Bengal has a characteristic of typical sub-tropical humid climate. Aggressive growth of weeds especially from several species Polygonum (Polygonum pensylvanicum, P. orientale, P persicaria) offered strong competition to the crops grown during the rabi season. Multiple nutrient deficiencies owing to coarse texture soil and high rainfall during the rainy season as well as high invasion of Polygonum become the major constraint in crop cultivation during the rabi season. Moreover, in sandy loam soil, pre-emergence application of pendimethalin often shows phytotoxicity to wheat plant in theses areas. Keeping this view in mind, an attempt has been made to work out the dose of pendimethalin in wheat for controlling Polygonum and to determine value of selectivity index at different moisture levels through bioassay technique. A field experiment was carried out during the rabi season of 2008 in the farm of Uttar Banga Krishi Viswavidyalaya. The cultivated wheat variety PBW-343 was used in the experiment. The soil of the experimental site was sandy loam in character with pH 5.42–5.8, organic carbon 0.45%, available nitrogen 110.50 kg/ha, available phosphorus 16.10 kg/ha and available potassium 77.40 kg/ha. In wheat, pendimethalin with the dose of 0, 0.30, 0.50, 0.60, 0.70, 0.80, 0.90, 1.00 and 1.20 kg/ha was used in the field experiment having the plot size of 2 sq m area at different moisture levels (33.92, 24.64 and 18.46%). In Polygonum sp., pendimethalin doses of 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45 and 0.50 kg/ha were used. Visual observations were made everyday to understand the changes in growth behaviour of the plants and appearance of phytotoxic symptom owing to herbicidal toxicity on plant at different doses. Both wheat and weed plant samples were taken at 25 days after sowing from 50 x 50 cm sample area. Biological response of plants to herbicides was determined by measuring the dry weight of the plants grown in herbicide treated soil with different doses and per cent growth inhibition of plant was calculated by comparing the dry weight of herbicide treated plant with healthy plant from untreated plot. The per cent growth inhibition values obtained at different herbicidal doses both in case of wheat and weed were transformed to probit values and regressed against log values of doses. The linear regression equation was computed by using excel programme Y=bx+a, where Y indicates probit value of % dry weight reduction, b is regression coefficient, x is log dose of herbicide and a is intercept of Y. Selectivity index (S. I.) value was calculated by using the following formula: